552 SEISMIC METHODS [Chap. 9 



the reluctance type is similar to phonograph pickup, the capdcitive detector 

 is built in the same manner as a condenser microphone, and the pressure 

 detector follows the design of the carbon microphone, or that of the 

 crystal microphone. All electrical pickups have in common a spring- 

 suspended mass whose motion relative to the instrument frame is converted 

 into electrical impulses by some sort of transdiMcr. The two component 

 parts of the transducer are attached to the seismograph mass and to the 

 frame. In the inductive detector, the transducer is a coil moving in a 

 magnetic field (see Figs. 9-1 13a and 9-1 13b). In the reluctance detectors 

 (Fig. 9-115), the transducers consist of iron armatures surrounded by coils 

 and placed close to the poles of a permanent magnet or magnets which 

 usually act as the detector mass. In the capacitive type of seismograph 

 (Fig. 9-116), the transducer is a condenser; the mass is mounted close to 

 a stationary plate so that the two together act as a variable condenser. 

 In the piezoelectric detector (Fig. 9-118), the seismograph mass rests on a 

 stack of quartz plates or a rochelle salt crystal. 



(6) Amplifier. Virtually all reflection equipment employs, between 

 seismograph and recording device, amplifiers of widely varying construc- 

 tion. At present the preference seems to be for the resistance-coupled 

 type, usually of three stages. The following features are common to most 

 amplifiers: filter systems, input and output transformers for matching 

 the impedance of the pickup and of the indicating device, battery operation, 

 gain and filter controls, A.V.C. systems, and separate volume expanders 

 (companders). A scheme of a seismic amplifier (without A.V.C.) is given 

 in Fig. 9-77. 



(c) Recorder. The recorder is a combination of recording camera and 

 a bank of galvanometers which may be of the coil, bifilar oscillograph, 

 or string type. The coil galvanometers are modified d'Arsonvals with 

 torsion wire or ribbon instead of jewel suspension and short natural 

 period. Therefore, they are really damped vibration galvanometers, 

 with oil or electromagnetic damping. Bifilar oscillographs are less 

 frequently employed than are coil galvanometers. They combine the 

 advantages of high sensitivity with high natural frequency, but greater 

 care has to be devoted to the design of a good optical system. String 

 galvanometers (harp of strings in a magnetic field) share most of the ad- 

 vantages of the oscillograph but have the drawback of shadow photography 

 (which is somewhat tiring for office work on the records), low sensitivity, 

 and possibility of tangling of the strings. 



{d) Time Marking. Since for the timing of reflection impulses an ac- 

 curacy of the order of one one-thousandth of a second is required, it has 

 become general practice to project time lines at one-hundredth of a second 

 intervals on the paper and interpolate to an accuracy of one-tenth line. 



